Adolescent girls with anorexia nervosa (AN) exhibit hormonal abnormalities that predispose to alterations in bone turnover, accrual, and modeling that can ultimately lead to irreversible skeletal deficits. The current investigators have a longtime interest in how hormonal alterations in this disease lead to changes in bone marrow composition and skeletal losses. The proposed project builds on a recent double-blind, placebo- controlled trial of dehydroepiandrosterone (DHEA) and estrogen replacement therapy (ERT) in adolescents with AN. The current investigators have shown that this combined anabolic + antiresorptive regimen mitigated bone loss of the axial skeleton as evidenced by dual-energy x-ray absorptiometry (DXA) measures of the spine and whole body. There is interest in whether this therapeutic regimen also affects the appendicular skeleton, a question of high clinical importance as young adolescents fracture extremities much more commonly than the axial skeleton. Preliminary data from the current group have also shown that adolescents with AN exhibit shifts from red to yellow (fatty) marrow with progression of disease and develop increased marrow fat. These young women, who paradoxically have depleted subcutaneous fat, have increased fat within bone marrow, with potential adverse long-term consequences for bone formation, bone accretion during adolescence, and ultimately, lifetime skeletal health. Further research is needed to determine whether this conversion of red to yellow marrow in adolescents mirrors findings in adults indicating an associated decreased bone density and increased fracture risk. This proposal seeks to investigate this question through a novel clinical trial of combined therapy with DHEA+ERT on bone mineral density (BMD) and bone marrow composition, examining the role of hormonal factors in the conversion of red to yellow marrow and relation of findings to appendicular BMD. Over the one-year trial, state-of-the-art imaging techniques will be employed, including peripheral quantitative computed tomography (pQCT), visual assessments of magnetic resonance imaging (MRI) data, MR relaxometry and magnetic resonance spectroscopy to measure bone mineral density and evaluate bone marrow composition, respectively. We also seek to understand hormonal mediators of the changes observed in both bone density and bone marrow composition, including adrenal and gonadal steroids, insulin-like growth factors, growth hormone, and ghrelin, adiponectin, and leptin. The proposed project will test hypotheses raised by the pilot imaging data, in particular whether hormonal abnormalities in girls with AN influence mesenchymal stem cells to differentiate preferentially into adipocytes over osteoblasts. Enhancing understanding of the mechanisms underlying skeletal losses in this young patient population has the potential to fill critical knowledge gaps that will improve the care of adolescents with AN. Knowledge gained from this study may also have application to other diseases across the age spectrum that are associated with bone loss and involve alterations in bone marrow composition.